There are many different types of display devices which can visually display information such as figures, numbers and video information. These devices include the ubiquitous cathode ray tube in which a raster is created by repetitively sweeping an electron beam in a rectangular pattern. The image is created by selectively modulating the beam to generate light and dark spots on the raster.
Another display device is an electromechanical scanning system in which a line of light-emitting devices is modulated with the information to be displayed. The illuminated line is converted into a raster by means of an oscillating mirror thereby generating a virtual raster image. These latter devices have the advantage that a full "page" display can be created from a much smaller number of light-emitting devices than is necessary to generate a normal full page real image.
In operation, an enlarged, virtual image of the illuminated devices is reflected from a mirror as the mirror is being physically pivoted about a fixed axis by means of an electromagnetic motor. Although it is possible to directly drive the mirror to produce oscillations, in order to reduce the power necessary to drive the mirror, it is possible to use a resonant electromechanical oscillator to move the mirror. In such an oscillator, the mirror is mounted on a spring attached to a frame so that the mass of the mirror and the spring create a mechanical resonator. An electromagnetic motor oscillates the mirror mass at the resonant frequency of the spring/mirror system. In this manner, only a small amount of power is needed to produce a relatively large oscillation. Such a conventional resonant oscillator is shown in U.S. Pat. No. 4,632,501 in which the mirror is attached to the base by a thin sheet of spring material.
A problem with the conventional mirror/spring oscillator system is that the rapid angular oscillation of the mirror requires a large spring force to accelerate and decelerate the mirror. The spring force is also applied to the base of the device and constitutes a "reaction force". When the base is rigidly secured to a relatively massive object, this force is not a serious concern. However, when it is impossible or undesirable to attach the display device to a massive object, as is the case for hand-held, eyeglass-mounted or "heads-up" displays, the force causes vibrations which are, at best, annoying and, in some cases, may cause the resulting image to be blurred or even unintelligible. In addition, the vibration can disrupt the function of an accompanying instrument, such as a microscope, that is sensitive to vibration. Further, even if the vibration is acceptable, the power required to oscillate the mirror increases when the vibration is transmitted to an external structure. This extra power means a larger motor is required to insure that the motor can drive the display with sufficient amplitude, in turn, resulting in increased battery drain for portable displays.
This problem is even more serious if the system design requires that the mirror pivot about a point near the end of the mirror as opposed to near the center of the mirror. Such a design is desirable in a hand-held display application because it permits use of smaller lenses and results in a more compact display.
Accordingly, it is an object of the present invention to provide a resonant scanning unit for an optical display device in which the net reaction force transferred to the mounting base is reduced.
It is a further object of the present invention to provide a resonant scanning unit which reduces vibration in a resonant-scanning optical display device.
It is yet another object of the present invention to provide a resonant scanning unit which allows the electromagnetic motor which drives the mirror to operate in an efficient manner.
It is yet another object of the present invention to provide a resonant scanner construction which uses a counterbalanced mass construction to reduce the net reaction force transmitted to the mounting base.
It is still a further object of the present invention to provide a resonant scanning unit in which the pivot point about which the mirror oscillates is located at a distance from the mirror center.